1. Field of the Invention
This invention relates to an information reproducing method for reproducing information recorded on an optical recording medium by using a laser beam, and to an apparatus for executing this method.
2. Description of the Related Art
In an optical information recording/reproducing apparatus for reproducing information marks recorded on information tracks of an optical recording medium using a laser beam, the laser beam is condensed as small as possible on the optical recording medium by using an objective lens. The minimum diameter of the optical spot formed by this means on the optical information recording medium is defined substantially as λ/NA by the wavelength A of the laser beam and the numerical aperture NA of the objective lens. In order to improve the recording density of the optical recording medium, on the other hand, the arrangement gaps (mark pitch) of the information marks in the optical spot scanning direction may be reduced. When the mark pitch becomes smaller than the spot diameter, however, the optical spot radiates simultaneously parts of other adjacent information marks when it radiates a target information mark. Therefore, signals of the adjacent information marks leak to the signal of the information mark that is to be reproduced (this leak will be hereinafter referred to as “inter-symbol interference”). This interference interferes with noise components and lowers reproduction accuracy. In a system that includes a laser having a specific wavelength and an objective lens, the interference of the signals of the adjacent information marks renders a critical problem for achieving the high density.
A method that applies a wave form equalization processing to a reproducing signal and reduces the inter-symbol interference has been employed in the past as means for lowering the mark pitch. Hereinafter, this equalization processing method will be explained with reference to FIG. 7 that schematically shows the wave form equalization processing. A reproducing signal 104 is inputted to an amplitude adjustment circuit 500-1 and to a delay circuit 510-2. The amplitude adjustment circuit 500-1 multiplies the reproducing signal 104 by a predetermined multiple in accordance with the equalization coefficient signal 502-1 outputted from a coefficient generator 501. When the equalization coefficient signal 502-1 is C1, for example, the reproducing signal 104 is multiplied by C1 by a multiplication circuit 505-1 contained in the amplitude adjustment circuit 500-1, and is outputted as a signal-after amplitude adjustment 520-1. On the other hand, the reproducing signal 104 inputted to the delay circuit 510-2 is delayed by a predetermined delay amount and is converted to a signal-after-delay 511-2. The equalization processing comprises a plurality of processing as shown in FIG. 7, and is therefore executed serially. In consequence, signals-after-amplitude adjustment 520-1 to 520-n, each receiving an intrinsic delay amount and an intrinsic amplitude change, are acquired. These signals-after-amplitude adjustments 520-1 to 520-n are added by an addition circuit 530 and a signal-after-equalization 108 is outputted consequently. If the equalization coefficient signal 502-1 to 502-n outputted from each coefficient generator 501-1 to 501-n is set in advance to an appropriate value, the amount of the inter-symbol interference contained in the signal-after-equalization 108 can be drastically reduced. These equalization coefficients and delay amounts are set in most cases to optimum values that are determined experimentally. Incidentally, when n=3, the processing is referred to as “3-tap equalization processing” and when n=5, “5-tap equalization processing”.
Incidentally, explanation of reference numerals 500-2, 502-2, 500-n, 502-n, 510-n and 511-n will be omitted because it is the same as the explanation of the reference numerals 500-1, 502-1, 510-2 and 511-2.
The diameter of the optical spot used for reproduction is defined substantially as λ/NA by the wavelength λ of the laser beam and the numerical aperture NA of the objective lens, as described above. In this case, the highest temporal frequency that can be reproduced is (4×NA)/λ. As the frequency of the highest density repetition signal recorded approaches the temporal frequency, the signal amplitude in reproduction becomes smaller, and reproduction becomes more difficult. Therefore, when the high density is achieved by reducing the mark pitch, the highest density repetition signal involves deterioration of a signal-to-noise ratio (S/N) resulting from the drop of the amplitude, and reproduction accuracy drops.
The amplitude of the highest density repetition signal can be increased generally when the inter-symbol interference is reduced by the equalization processing described above. In consequence, the S/N can be improved. However, when a higher density is attained by reducing further the mark pitch, the wave form equalization system cannot acquire a sufficient S/N improvement effect while reducing the inter-symbol interference. The result is shown in FIGS. 6A-6E. FIGS. 6A-6E show the simulation result of the reproducing signals in accordance with the Hopkins' diffraction calculation described in “J. Opt. Soc. Am.”, Vol. 69, No. 1, January (1979), pp 4-24, that executes the simulation of the optical disk reproduction process in consideration of optical diffraction due to the information marks and the numerical aperture NA of the objective lens. This simulation assumes an 8-16 modulation system using a light source wavelength of 660 nm, an objective lens numerical aperture NA of 0.6, and a recording linear density on tracks of 28 μm/bit. Since a window width (Tw) is 0.14 μm in this case, the highest density repetition signal is recorded as a repetition of a pattern comprising a recording mark having a length of 0.42 pm and a non-recorded portion having a length of 0.42 pm.
FIG. 6A shows the display of the eye pattern of the reproducing signals before processing. It can be appreciated that opening cannot be obtained sufficiently in the proximity of the slice level (level “0”) due to the inter-symbol interference from the preceding and subsequent recorded marks. FIG. 6B shows the eye pattern as a result of the equalization processing of this signal. The equalization processing uses 3-tap equalization processing of n=3. The coefficients are set to d1=d3=−0.12 and d2=1.0 and the delay amount by the delay circuit is twice the window width. In consequence, the inter-symbol interference can be reduced. It can be appreciated that opening of the eye in the proximity of the slice level becomes greater than in FIG. 6A. However, the amplitude of the highest repetition signal is about ⅓ of the amplitude of the highest density repetition signal, and a sufficient S/N cannot be obtained. FIG. 6C shows the eye pattern when the coefficients are set to d1=d3=−0.30 and d2=1.0, and the amplitude of the highest density repetition signal is increased. In this case, the edge shift becomes great, and opening in the proximity of the slice level becomes small, on the contrary, though the amplitude of the highest repetition signal becomes great. As described above, the wave form equalization processing system cannot obtain a sufficient S/N improvement effect while reducing the inter-symbol interference, and the problem encountered in achieving the high density by reducing the mark pitch remains yet unsolved.